The present invention relates to a method for processing dusts and muds from dust removing plants in the iron and steel industry where the dusts are pelletized for further processing.
During the dust removal from waste gases of metallurgical plants, such as blast furnaces, converters, electrofurnaces and sintering plants, a large quantity of dust-like matter is produced which should be dressed and further processed, particularly due to its considerable iron content.
Dust-like matter may develop, depending on the type of dust removal process employed, either in dry dust removal plants as a dry dust, or in wet dust removal plants as a water-dust dispersion. The water-dust dispersion can be concentrated in such a manner that a mud is deposited from the dispersion in a thickener while clear water flows out through an overflow of the thickener.
In the production of raw iron, 5 to 20 kg dust are developed in blast furnace dust removal plants per ton of raw iron. In the production of steel in LD converters, 18 to 21 kg dust are developed per ton of steel. The exact quantities of dust developed depend on the size of the metallurgical plant, the raw materials employed, and the mode of operation involved.
The use of dusts and muds in sintering systems is very limited due to the very fine consistency of the solid matter and due to the varying contents of zinc, lead and alkalis which have an adverse influence on blast furnace operation. Dumping of such substances is sometimes also not possible due to environmental protection laws.
Due to the relatively high iron, zinc and lead contents, these substances are dressed and processed either as dry dust, mud or a mixture of the two. One prior proposal for treating the dust material provides for pelletizing the dust material together with iron ore concentrate to form green pellets. The green pellets are then prehardended on a traveling grate and thereafter are reduced in a series-connected cylindrical rotary kiln with the addition of solid fuels whereby zinc and lead volatilize. The resulting products are metallized pellets containing relatively small quantities of zinc and lead. These metallized pellets, however, have a relatively high sulfur content so that use thereof is possible mainly in blast furnaces. There further exists the danger of the green pellets sintering together on the traveling grate since carbon is always present in the blast furnace dusts. To avoid such sintering together, it is necessary, in the above-mentioned processes, to add relatively large quantities of fine ore as a leaning material to the mixture which is formed into the green pellets, or to prevent that oxigen may enter into the grate area.
In the past, binders, such as bentonite, calcium hydroxide, starch, limestone, and other similar materials, have been used in the production of pellets. In addition, oil and pitch have been used as binders.